EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Buddhi19 on September 15, 2023, 03:22:13 am
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I'm using TLC4502D IC to amplify the PH sensor output as in the diagram. It was observed that the output is lower and not constant( not converging) for the same PH sensor probe when I tested with multiple ICs of the same type. Is there any reason for this improper amplification?
A reference voltage of 1024 mV was given by LM4140 IC to the pin number 5. R7 is 20k and R6 is 10k. I want to have the output in the range 2.8-3.7V (when the prob is connected to some TLC4502D it gives an output in the required range but some don't. I want to find the reason for that)
I want to have the output of the IC in a fixed range and should converge quickly.
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By converge, do you mean "settle" to a final value?
I'd be looking for sources of parasitic capacitance and leakage currents at the opamp inputs.
A teraohm of input impedance needs to be treated with the utmost respect.
Your components and pcb need to be scrupulously clean. No dirt, flux residues or finger prints. It will fail to work as hoped as a breadboarded circuit.
Use guarding techniques. microchipdeveloper.com/opamp:leakage-currents
Is the opamp oscillating? Sensor and cable capacitance at the non-inv input can cause stability issues. If it is, add a small cap across R7. Guesstimate or measure your cable plus sensor capcitance. The feedback cap needs to be at least that value. pH sensor cables have a typical capacitance of 50-100pF per metre.
You can neutralise this capacitance with a bootstrapped shield. A "driven shield" is a better option.
The shield driver opamp doesn't need to be anything special. The resistors keep the feed back below unity, that stops the bootstrap causing oscillation.